1. Field of the Invention
This invention relates to the field of leg locking and supporting systems for self-elevating platforms or jack-up rigs of the type used in the offshore exploration and production of hydrocarbons, as well as for other purposes. Offshore platforms have been used extensively by the oil and gas industry in continental shelf regions for oil and gas drilling, production, operations, pipeline pumping stations, personnel accommodations and miscellaneous service and work-over operations.
Fixed offshore platforms, intended to remain in one location, traditionally are built on shore, transported by barge to the offshore location, launched and rotated into an upright position and permanently affixed to the sea floor. Mobile offshore vessels have been developed to meet the offshore industry's needs for a facility from which drilling, production or work-over operations can be conducted and which usually will remain at one location only while operations are conducted, after which it can be moved to a different location. Various types of mobile offshore vessels have been developed to meet the needs of the industry including semisubmersible platforms and floating drill ships for deep water operations, posted barges for inland waters or bayous and jack-up platforms for shallow to moderate water depths.
The usual jack-up offshore drilling rig or platform includes a barge hull and supporting legs which are capable of being operated to raise the hull above the surface of the water. The barge hull may be towed as a floating vessel from one location to another with the legs raised up through the hull. Upon reaching the intended location, the elevating system will lower the legs through the barge hull until firmly engaged with the sea floor. Continued downward jacking on the legs will result in penetration of the legs into the sea floor until a firm foundation for the footings is achieved, after which, continued jacking will cause the hull to lift above the sea surface to a height greater than the anticipated highest wave height during operations.
The elevating systems for jack-up rigs conventionally include three or more legs, each leg consisting of one or more chords, but most typically of three chords. One or more gear racks extend longitudinally along the length of the chords of each leg and are driven by pinion gears attached to the hull and powered by hydraulic, electric or electro-mechanical means in a manner well known to those skilled in the art. The pinion gears may be arranged such that the pinion teeth face the center of a trussed leg with multiple chords, or they may be oriented as opposed pinions with a toothed rack mounted on each side of a leg or leg chord to engage the opposing pinions. Multiple pinions often are stacked vertically to provide enough force to lift the desired loads.
2. Description of the Related Art
Such jack-up rigs are subject to large environmental loadings from storms which exert wind forces on the platform and wind and wave forces on the legs of the platform. A combination of these forces, together with the heavy weight of the platform, can result in a large interaction force between the platform and the legs which must be resolved at the leg-to-hull interface or connection. To assist in strengthening and rigidifying the leg-to-hull interface, jack-up rigs typically are provided with leg locking systems which are engaged after the platform has been elevated to its desired position or, in some cases, when storm conditions are anticipated. Prior art leg locking systems typically include elongated chocks which have surfaces configured to conform to the teeth on the elongated leg racks. The chocks are positioned vertically so as to mesh with the teeth and then are moved horizontally by means of hydraulic cylinders, screw jacks, electric motors, etc. until they firmly engage a plurality of teeth on each chord of each leg. Various types of mechanical and hydraulic means then are used to lock the chocks into engaged position, so that they serve to lock the legs in position, as well as to rigidify the elevated structure and insulate the pinion gears from stress loading due to storm waves and the like.
A principal problem in such prior art structures relates to the necessity for properly vertically aligning the toothed chocks and the teeth of the racks on the leg chords prior to engaging the chocks. The pinion gears can position the legs vertically. However, since the legs are large, the individual rack teeth at the three leg apexes may vary slightly from each other in vertical relationship to the surface of the hull, due to manufacturing tolerances, imposed loads and similar factors. It is not unusual, with the leg at a set position, for rack teeth at one apex of the same leg to vary vertically, relative to the hull, from those of another apex of the same leg by 1 to 3 inches, plus or minus, over the 12 inch vertical dimension of a typical tooth. Thus, mating engagement of the chocks with the leg rack teeth requires that means be provided for limited vertical adjustment of the individual chocks relative to the platform body, so as to align the teeth of each chock with the teeth of each of the leg racks prior to mating engagement of the chock teeth with the rack teeth. Various prior art leg locking systems have provided this function by including means for vertical adjustment of the chocks relative to their supporting housings or structures mounted on the rig hulls, after which the chocks are locked in their vertical position prior to horizontal engagement of the chock teeth with the rack teeth. With such systems, if the vertical adjustment of the chocks is imprecisely done, a slight vertical misalignment between the chock teeth and leg rack teeth can result, which will produce stress concentrations between partially engaged teeth which greatly reduce the effectiveness of the chocks.
Another problem presented by prior art leg locking devices is their failure to accommodate manufacturing tolerances of leg rack teeth. Most rack teeth for jack-up rig legs are flame cut out of heavy steel plate guided by a physical template or computer control. Cutting heat and subsequent heat treatment can cause distortions, producing teeth which can vary in size by as much as 1/8 inch over a typical 12 inch tooth. Since it is desirable, in leg locking systems, to have the toothed chocks engage at least four teeth of each leg rack, the accumulation of manufacturing tolerance errors over the length of four teeth can be enough to cause improper mating of some of the teeth, again causing stress concentrations which negate the desired even distribution of loading forces over the engaged teeth.
A further problem with most prior art leg locking devices is that the devices, after being exposed to storm loadings, may become jammed and are very difficult to disengage when it is desired to release the leg locking systems.
In addition, some prior art systems rely upon hydraulic forces for retaining the chocks in mating engagement with the leg racks, which creates a risk of disengagement in the event all power is lost on the platform.